Wouldn't it also be for the very simple fact that China in that period was in a very weak position and Japan just walked in without my resistance. China since the last emperor up to the rise of Mao was in a huge turmoil between various fractions trying to seize power, same time lagged very behind in technology thus let's face wasn't much of an opponent towards Japan that was rather industrialized.

Yes Japan committed all over SE Asia atrocities, though your statement is questionable who was the worst. Though this isn't much relevant Japan simply walked over all countries while Germany committed itself serious war on the East and West facing some rather formidable opponents resulting in far more clashes.

Further more why is the history regarding that period so limited, in the West we learn about what goes on in the West. We had 2 year history classes about what went on. Though like myself being Dutch we learned rather little about what went on in Asia and Indonesia specifically. (Which isn't to surprising either considering we committed quite some atrocities over there was well which we rather don't know about).